1. Field of the Invention
The present invention relates to a colored light-emitting device which can serve as an illumination or a light source for an indicator, and more particularly to a light-emitting device having multiple active layers.
2. Description of the Related Art
Since a light-emitting device having AlGaInP as a light-emitting layer is at least one digit brighter than a conventional colored light-emitting device, demands in use applications different from a conventional light-emitting diode, e.g., an in-car illumination or an LCD backlight are expanding. Although a fact that AlGaInP is of a direct transition type also contributes to an increase in intensity, enhancement of external quantum efficiency by providing a transparent and thick window layer is also a factor of the increase in intensity.
On the other hand, for example, Non-patent Literature 1 discloses that internal quantum efficiency is enhanced by providing a thick transparent conductive layer to a substrate and a window layer and also providing a multiple quantum well (MQW), thereby improving light-emitting efficiency.
In an AlGaInP-based light-emitting device, AlGaAs or GaP is used for a window layer. An AlGaAs layer has a problem in characteristics that it is degraded with respect to moisture, and GaP is generally used. However, to provide a thick GaP layer, a GaP substrate must be directly bonded to the AlGaInP light-emitting layer, or a thick film of GaP must be subjected to crystal growth. According to a method for directly bonding the GaP substrate, for example, as disclosed in Patent Literature 1, a barrier layer may be disadvantageously produced at a bonding interface with respect to the GaP substrate, and a long-time and high-temperature heat treatment is required to avoid this problem.
Further, although providing the window layer on one surface of the light-emitting layer is effective for improvement in light-emitting efficiency, it has been known that also providing the window layer on the other surface, i.e., providing it on both upper and lower surfaces of the light-emitting layer results in further improvement in external quantum efficiency. In this case, the window layer provided on the other surface is also formed by bonding or crystal growth, since a GaAs substrate functions as a light absorption layer, the substrate must be removed before forming the window layer.
A layer structure made of an AlGaInP-based material required for the light-emitting device is generally formed on a GaAs substrate by a MOVPE method. A total film thickness is no more than approximately 10 μm. Although an AlGaInP-based material and a GaAs-based material are lattice matching materials, a selective etching method can be used for these materials, and hence appropriately inserting a layer required for the selective etching between the GaAs substrate and an AlGaInP layer enables removing the GaAs substrate with etching.
However, a total film thickness of the AlGaInP-based material required for forming a functional layer which is necessary for light emission is approximately 10 μm at most and, if the GaAs substrate is removed and the AlGaInP layer alone remains, a film thickness of a remaining wafer is approximately 10 μm. Although the wafer having a film thickness of approximately 10 μm can be experimentally handled, it can be easily broken and does not have mechanical strength required to pass through an industrial process.
Therefore, a method by which strength holding plate (or a wafer) configured to hold mechanical strength is bonded to an AlGaInP growth surface side before removing a GaAs substrate and then the GaAs substrate is removed, is also considered. In this case, after a GaP substrate is bonded to a surface side from which the GaAs substrate has been removed, the strength holding plate (or the wafer) must be delaminated (removed), thereby cleaning is required along with the delamination, or there is a concern about contamination, costs are industrially increased, and there are not many merits. Therefore, to pass a wafer through an industrial process while saving costs, a method for providing the wafer with the mechanical strength by subjecting a thick GaP layer to crystal growth before removing the GaAs substrate is rational since the GaP layer portion can function as both a light extraction layer and the strength holding plate.
In case of subjecting the thick GaP layer to crystal growth in this manner, a thickness required for providing the mechanical strength sufficient to pass through the industrial process is not lower than 20 μm. To form a GaP layer having a film thickness of not smaller than 20 μm by crustal growth, several to ten-odd hours are required. Since the external quantum efficiency is enhanced as the film thickness of the GaP layer is increased, a long growth time is required. Furthermore, as a temperature required for growth of the GaP layer, a high temperature equal to or higher than a temperature necessary for growth of the AlGaInP layer is generally required, and the AlGaInP light-emitting layer portion is exposed to a temperature at the time of MOVPE growth or a temperature that is higher than the temperature of MOVPE growth for a long time.
A p-conductivity type cladding layer is doped with p-type impurities such as Mg or Zn, these impurities are heated at the time of the above-described crystal growth, thereby they diffuse based on thermal dynamics, and they may also possibly diffuse in a active layer. Since the p-type impurities that have diffused in the active layer are apt to form a defect, a defect is formed during a device life test by energization or the like, a reduction in carrier injection efficiency or an increase in light absorption thereby occurs, and an optical output reduction phenomenon is caused during the device life test.
The diffusion of the p-type impurities is largely dependent on Al composition x in (AlxGa1-x)yIn1-yP, the impurities rapidly diffuse if a mount of the Al composition x is small, and hence the impurities hardly stay. For example, the active layer contains less Al composition x, an impurity diffusion speed in the active layer is relatively higher than that in the cladding layer having a large amount of Al composition x, and the impurities hardly stay. Although impurity concentration varies depending on impurity concentration in an adjacent layer, the layer adjacent to the active layer requires the cladding layer for carrier confinement, and the cladding layer is generally doped. Since the cladding layer requires a wider band gap than that of the active layer, an amount of Al composition x is large, and thereby the impurity diffusion speed is lower than that in the active layer. Moreover, to prevent the injection efficiency with respect to the active layer from lowering, the cladding layer must hold the impurities whose concentration is not lower than a given level. Therefore, the impurities present in the cladding layer diffuse into the active layer.
If the active layer has a thickness more than a certain extent even though the impurities diffuse, a photo-activating portion can be designed with impurity concentration lower than impurity concentration that causes an influence of the impurity diffusion. For example, when a portion where a defect is formed due to the impurity diffusion to the active layer has a thickness of approximately 50 nm and an effective active layer film thickness required for radiative recombination is approximately 500 nm, providing a homogeneous active layer having a thickness of approximately 550 nm and a uniform combination enables maintaining the radiative recombination in the active layer even though impurities diffuse. However, this impurity diffusion contaminated layer having the thickness of approximately 50 nm is a layer having non-radiative recombination higher than that in any other active layer, and this layer can be a cause of a reduction in luminous efficacy. This type of active layer will be referred to as a bulk type active layer for the convenience's sake.
As described above, although the bulk type active layer is an active layer that is advantageous to suppression of an influence of the impurity diffusion, only the carrier confinement effect sandwiched between p-type and n-type cladding layers can be expected, a region contaminated with the impurities has a function as a non-radiative recombination layer, and hence the luminous efficacy is hardly improved. The bulk type active layer has internal quantum efficiency of approximately 60% at most, and the internal quantum efficiency is required to be enhanced.
As a method for enhancing the internal quantum efficiency, for example, as disclosed in Patent Literature 2 or the like, there is a method using a multiple quantum well (MQW) structure. When the MQW structure is adopted, the luminous efficacy can be improved by the confinement effect relative to a quantum well. However, since a thickness of each layer in the MQW is close to a de Broglie wavelength of an electron in a semiconductor, i.e., several to more than 10 nm, a thickness of each layer is very smaller than that of the bulk active layer. Therefore, as described above, an influence of the impurity diffusion to the active layer becomes considerable. This problem may be possibly solved by increasing the number of active layers in the MQW, but the number must be greatly increased, and the internal quantum efficiency is lowered due to self-absorption of the active layers.
Additionally, there is also a method, which is similar to the MQW, for setting each layer to a film thickness that is equal to or higher than the de Broglie wavelength and enhancing the luminous efficacy with the smaller number of layers. In this case, since the impurity diffusion is appropriately controlled, a problem hardly occurs during a device life test, and a light-emitting device with long life duration can be fabricated.
Even in a material other than the AlGaInP-based material Mg diffusion suppressing effect is shown by sandwiching a layer having a different composition, and this effect can be found in Patent Literature 3 and others.
However, when a film thickness equal to or higher than the de Broglie wavelength is adopted, since tunneling does not occur in a barrier layer provided between active layers, carrier transport from the active layer to another adjacent active layer has nothing to depend on except pumping. Since an electron has a small effective mass, the pumping is relatively easy, an effective mass of an electron hole is greatly larger than that of an electron, a statistical probability of the pumping that exceeds the high barrier layer is lower than an that of an electron, and hence the carrier injection efficiency in the active layer and the luminous efficacy are decreased in especially a low-current region with a small number of carriers. Additionally, a series resistance component is increased due to a reduction in carrier injection efficiency. This effect can be a serious problem in a device to be used in a low-current region like a light-emitting diode. However, a state that the pumping of carries hardly occurs means that the carrier confinement effect is improved, and the luminous efficacy is increased by effect of carries confined in the active layer.
The same effect as the above description that the series resistance component is increased by inserting a layer made of material with a wider band gap than that of the active layer is disclosed in, e.g., Patent Literature 4.
As a method for solving the above-described problem, in a configuration that thick transparent layers are provided on upper and lower sides of the light-emitting layer and an active layer and a barrier layer are alternately laminated, as disclosed in Patent Literature 5, a band gap of the barrier layer on a p-type side is decreased to reduce a VF (a forward voltage), and thereby a light-emitting device having high luminance and long life duration can be obtained. However, it is insufficient to solve a problem of a reduction in luminance and others, and a device with higher quality has been demanded.
Patent Literature 1: Japanese Unexamined Patent Application Publication No. 2006-32837
Patent Literature 2: Japanese Unexamined Patent Application Publication No. 2003-46200
Patent Literature 3: Japanese Unexamined Patent Application Publication No. Hei 06-283825
Patent Literature 4: Japanese Unexamined Patent Application Publication No. Hei 11-251687
Patent Literature 5: Japanese Unexamined Patent Application Publication No. 2010-087270
Patent Literature 6: Japanese Unexamined Patent Application Publication No. Hei 06-283822
Patent Literature 7: Japanese Unexamined Patent Application Publication No. Hei 06-310813
Patent Literature 8: Japanese Unexamined Patent Application Publication No. Hei 08-088404
Non-patent Literature 1: Applied Physics Letters Vol. 74 No. 15 pp. 2230-2232